Output power split hybrid electric drive system

ABSTRACT

A power split hybrid wheel drive system of the present invention comprises a planetary gear train ( 16 ) coupled to the output shaft of a driving engine ( 12 ). A primary motor or generator ( 18 ) is coupled to an output shaft of the planetary gear train ( 16 ) for receiving power therefrom, and a secondary motor or generator ( 30, 32 ) is associated with each driven vehicle wheel ( 14 ) to provide an electrical power pathway. A mechanical differential gear train ( 20 ) is coupled between the output shaft of the planetary gear train ( 16 ) and each driven vehicle wheel; ( 14 ) to provide a mechanical power pathway, while a mechanical clutch ( 22 ) and brake system ( 24 ) is configured to regulate the mechanical power routed to the differential gear train ( 20 ). A control system ( 48 ) is included to regulate the flow of electrical power between the primary motor ( 18 ) and secondary motors or generators ( 30, 32 ).

TECHNICAL FIELD

[0001] The present invention relates generally to a vehicle wheel drivesystem, and in particular to a vehicle wheel drive system having a powertransmission providing an infinitely variable speed ratio as well asindependent speed and torque control for each of driven vehicle wheel.

BACKGROUND ART

[0002] A typical vehicle features a driving engine, a power transmissionsystem, and a final drive with two output differential axles connectedto one or more sets of driven wheels. A conventional power transmissionsystem includes a single input shaft and a single output shaft, andusually operates with stepwise speed ratio changes defined by themechanical gear ratios contained therein. The power from the engine isdelivered through the input shaft to the power transmission system, andthen through the output shaft to the final drive. The final drivedistributes the driving power through each differential axle to eachdriven wheel on an equal-torque basis. To achieve a desired vehiclespeed, the engine speed is varied between each speed ratio change in thepower transmission system. The wide variation in driving engine speedreduces fuel efficiency and increases exhaust emissions.

[0003] When road surface variations at each wheel produce differentcoefficients of friction, the lower wheel driving torque of the twowheels limits the effective driving torque, which is twice the lowestwheel torque. The application of torque in excess of the lowest wheeltorque level results in spinning of the vehicle wheel.

[0004] Infinitely variable power transmission (IVT) systems andcontinuously variable power transmission (CVT) systems have beendeveloped to provide continuous speed ratio changes, thereby improvingvehicle fuel efficiency, driving comfort, and reducing vehicle exhaustemissions by permitting the vehicle engine to maintain a relativelyconstant speed. Both IVT and CVT systems are capable of providingcontinuous speed ratio change between the driving engine and drivenwheels, however, the output speed of an IVT can be reduced to zero andeven reversed, while the output speed of a CVT cannot. This IVTcapability is an important feature, because devices utilized to assistin vehicle launch (i.e. transition from a stopped state to a movingstate), such as torque converters or clutches, can be eliminated.

[0005] There are two types of IVT systems: hydro-mechanical IVT systemsand electro-mechanical IVT systems. Although a wide variety ofconfigurations are possible, the vast majority of IVT systems operate ona power-split concept. They feature a single input for receiving powerfrom the driving engine and a single output shaft for delivering thepower to the final drive and associated driven wheels. They furtheremploy some form of power splitting devices, allowing the power at theinput shaft to be converted partially or fully converted intonon-mechanical forms such as hydraulic power or electric power, and thenreconverted back to mechanical forms before leaving at the output shaft.

[0006] Hydro-mechanical IVT systems are known in the art to have severaldrawbacks. First, hydraulic drives are not suitable for high-speedoperation. This, to a large degree, limits hydro-mechanical IVT systemsto non-automotive applications. Secondly, hydrostatic pump or motorsutilized in hydro-mechanical IVT systems are very noisy when operated atpressures greater than 5,000 PSI. Finally, hydrostatic pumps and motorsare not conducive to shaft-concentric and compact transmission designs.In addition, the positions of the hydrostatic units such as pumps ormotors within the IVT system may be subjected to various mechanicalconstraints.

[0007] The electro-mechanical IVT systems overcome several of theaforementioned problems. Recent developments in electro mechanicalsystems have demonstrated, among other features, advantages efficiency,controllability, and system flexibility. Furthermore, with the additionof energy storage systems, electro-mechanical IVT systems can alsofunction as power regulators, buffering output power fluctuations,thereby providing vehicle drive system hybridization options

[0008] U.S. Pat. Nos. 5,907,191 5,914,575 5,991,683 and 5,920,160 eachassigned to Toyota Jidosha Kabushiki Kaisha of Toyota, Japan disclose asingle-node electro-mechanical power split transmission known in theindustry as the Toyota Hybrid System (THS). The THS employs a singleplanetary gear system comprising a sun gear, a ring gear, and aplanetary carrier as a power splitting device, such that the THS deviceis categorized as an output power split system. In the THS, theplanetary carrier is connected to the input shaft to receive power fromthe driving engine. The associated sun gear is connected to an electricmotor/generator. The ring gear is connected to a second motor orgenerator and to the output shaft that delivers the power to the drivingwheels through a differential. The THS has an adequate speed ratio rangefor compact passenger car applications. Within the transmission speedrange, there is a point where zero power passes through the electricmotor path. Power is transmitted mechanically from the input shaft tothe output shaft. This point is defined as the node point at which thetransmission yields the maximum efficiency.

[0009] While the THS is suitable for providing infinitely variable speedand some level of vehicle hybridization, the output power to eachdriving wheel cannot be individually controlled. When driving in uneventerrain having varied surface coefficients of friction, it is highlydesirable to match the driving power supplied to each individual drivenwheel to different driving requirements. Driving the driven wheels atdifferent speeds and individually controlling the driving torque whentraveling on a slippery surface or around a curve has the distinctadvantages of avoiding vehicle deformation, reducing tire wear,attaining improved traction, and enhancing vehicle dynamic stability.

[0010] U.S. Pat. No. 5,947,855 assigned to Deere & Company of Moline,ill. discloses a vehicle hybrid wheel drive system. The hybrid drivesystem includes a pair of summing gears, each having an output shaftcoupled to a respective driven wheel. Each summing gear also features afirst input coupled to a drive shaft and a second input coupled to arespective electric motor. The drive shafts in both summing gears areoperatively connected to a common shaft that in turn connects to drivinginternal combustion engine and an electric generator. This system has asingle node point where zero power passes through the electric motorpath, and since the torque splitting takes place at the power input,this system is classified as an input power split system. An input powersplit system is most suited for high-speed operation beyond the nodepoint because at slow vehicle speed operation, below the node point,excessive internal power circulation takes place within the powertransmission system. The power that passes through the electric path canbecome several times greater than the mechanical transmission power.This significantly reduces the efficiency, thereby offsetting thebenefit otherwise produced by the IVT system.

[0011] In addition, the mechanical system disclosed in U.S. Pat. No.5,947,855 is complex, having two sets of identical Ravigneaux compoundplanetary trains. This not only affects the compactness of the drivesystem, limiting the application scope, but also increases the cost.

[0012] Accordingly, it would be highly desirable to provide a simplepower split hybrid wheel drive system which is capable of providing aninfinitely variable speed ratio and of controlling both torque and powerapplication to multiple driven wheels of a vehicle, so as to provide forimproved vehicle fuel efficiency, reduced driving engine emissions, andenhanced vehicle dynamic stability.

SUMMARY OF THE INVENTION

[0013] An objective of this invention is to provide a simple power splithybrid wheel drive system capable of providing individual infinitelyvariable speed and torque, as well as drive power control to multipledriven wheels on a vehicle, and which is capable of maintaining adriving engine in a high fuel efficiency operational state over theentire vehicle driving range.

[0014] Briefly stated, a power split hybrid wheel drive system of thepresent invention comprises: (a) a planetary gear train coupled to theoutput shaft of a driving engine; (b) a primary motor or generatorcoupled to a shaft of the planetary gear train; (c) a secondary motor orgenerator associated with at least one driven vehicle wheel; (d) amechanical differential gear train coupled between the output shaft ofthe planetary gear train and driven vehicle wheels; a mechanical clutchand brake system to regulate the power routed to the differential geartrain; and (d) a control system to regulate the flow of electrical powerto and from the primary and secondary motors or generators.

[0015] In an alternate embodiment, the wheel drive system furthercomprises: (e) an energy storage system coupled to the control system;(f) a direction selection gear train associated with the mechanicaldifferential gear train; (g) two or more additional secondary motors orgenerators coupled to additional driven vehicle wheels; and (h) one ormore conventional speed reduction units.

[0016] The hybrid wheel drive system of the present invention providestwo power paths for power transmission. One is a mechanical path, fromthe driving engine through the planetary gear train to the mechanicaldifferential drive and to the driven wheels. The other is an electricpath in which the input mechanical power from the driving engine isconverted to electric power by one or more primary motors or generatorsand then regulated and transmitted through a control system to secondarymotors or generator. The latter then converts the electric power back tomechanical power, combining it with the power from the mechanical pathat the output or driven wheels.

[0017] The hybrid wheel drive system of the present invention providesinfinite speed ratio change, allowing the speed of vehicle to vary fromzero to maximum rated speed with high efficiency. Further, the hybridwheel drive system of the present invention provides at least one pointwhere no power is passing through the electric path, and all powertransmitted from the driving engine to the driven wheels passes throughthe mechanical path for maximum efficiency.

[0018] The drive system of the present invention permits individualcontrol to each of the vehicles' driven wheels, is capable of providingtraction control, and of providing active differential function to thevehicle. Furthermore, the wheels on the two side of the vehicle can bedriven in opposite direction if desired for special steering capability.

[0019] With optional direction selecting devices and clutches, the drivesystem can have the same drive range in forward and reverse operations.

[0020] The foregoing and other objects, features, and advantages of theinvention as well as presently preferred embodiments thereof will becomemore apparent from the reading of the following description inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the accompanying drawings which form part of thespecification:

[0022]FIG. 1 is a schematic diagram of the preferred embodiment of thedrive system of the present invention;

[0023]FIG. 2 is a graphical representation of engine, motor, andgenerator speed versus wheel speed for the preferred embodiment of FIG.1 under uniform traction conditions;

[0024]FIG. 3 is a graphical representation of engine, motor, andgenerator torque versus wheel speed for the preferred embodiment of FIG.1 under uniform traction conditions;

[0025]FIG. 4 is a graphical representation of engine, motor, andgenerator power versus wheel speed for the preferred embodiment of FIG.1 under uniform traction conditions;

[0026]FIG. 5 is a graphical representation of engine, motor, andgenerator torque versus wheel speed for the preferred embodiment of FIG.1 under uneven traction conditions;

[0027]FIG. 6 is a graphical representation of engine, motor, andgenerator power versus wheel speed for the preferred embodiment of FIG.1 under uneven traction conditions;

[0028]FIG. 7 is a schematic diagram of a first alternate embodiment ofthe drive system of the present invention including a drive directionselecting device;

[0029]FIG. 8 is a schematic diagram of a second alternate embodiment ofthe drive system of the present invention;

[0030]FIG. 9 is a schematic diagram of a third alternate embodiment ofthe drive system of the present invention;

[0031]FIG. 10A is a schematic diagram of a fourth alternate embodimentof the drive system of the present invention;

[0032]FIG. 10B is a sectional view of a first alternate embodiment speedreducer utilized in FIG. 10A;

[0033]FIG. 10C is a sectional view of a second alternate embodimentspeed reducer utilized in FIG. 10A;

[0034]FIG. 10D is a sectional view of a third alternate embodiment speedreducer utilized in FIG. 10A;

[0035]FIG. 11 is a schematic diagram of a fifth alternate embodiment ofthe drive system of the present invention;

[0036]FIG. 12 is a schematic diagram of the fifth alternate embodimentof the drive system shown in FIG. 11, further including speed reductionunits;

[0037]FIG. 13 is a schematic diagram of a sixth alternate embodiment ofthe drive system of the present invention;

[0038]FIG. 14 is a graphical representation of engine, motor, andgenerator torque versus wheel speed for the embodiments of FIGS. 12 and13 under uniform traction conditions;

[0039]FIG. 15 is a graphical representation of engine, motor, andgenerator speed versus wheel speed for the embodiments of FIGS. 12 and13 under uniform traction conditions; and FIG. 16 is a graphicalrepresentation of engine, motor, and generator power versus wheel speedfor the preferred embodiments of FIGS. 12 and 13 under uniform tractionconditions.

[0040] Corresponding reference numerals indicate corresponding partsthroughout the several figures of the drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

[0041] The following detailed description illustrates the invention byway of example and not by way of limitation. The description clearlyenables one skilled in the art to make and use the invention, describesseveral embodiments, adaptations, variations, alternatives, and uses ofthe invention, including what is presently believed to be the best modeof carrying out the invention.

[0042] Referring to FIG. 1, there is shown a first embodiment of thedrive system 10 of the present invention, coupled between a drivingengine 12 and a set of driven wheels 14. The drive system 10 comprisesplanetary gear train, indicated generally at 16, driven by an outputshaft 17 from the driving engine 12. Output from the planetary geartrain 16 is split between a primary electric motor (generator) 18 and amechanical differential gearbox 20. Output from the planetary gear train16 is regulated by a mechanical clutch 22 and a brake system 24.

[0043] Output from the mechanical differential gearbox 20 is directedalong a pair of intermediate output shafts 26 and 28, to secondaryelectric motors (generators) 30 and 32. The output shafts 26 and 28 arecoupled to the secondary electric motors (generators) 30 and 32 eitherdirectly or via one or more conventional gear trains (not shown). Eachsecondary electric motor 30, and 32 drives an associated driven wheel 14through an associated wheel output shaft 34 or 36. The output shafts 26and 28 are also mechanically coupled to wheel output shafts 34 and 36,respectively. The driven wheels 14 are each coupled to the associatedwheel output shafts 34 or 36 either directly of via a conventionalreduction gear train (not shown).

[0044] It should be noted that in the context of this invention, theterm “wheel” is not limited to a pneumatic tire and rim combination asfound on conventional automotive vehicles, but will be understood bythose of ordinary skill in the field to include sprocket drives andtracked wheel system. Similarly, the term “motor” as used herein, isintended to describe an electrical device which is capable of convertingreceived electrical power into mechanical power, i.e. having a motorstate, and is further capable of converting received mechanical powerinto electrical power, i.e. having a generator state.

[0045] It should further be noted that while the present invention isdescribed in the context of an electrical-mechanical drive system 10,some of the principles, arrangements, or drive system configurationsdisclosed herein are equally apply to hydro-mechanical drive systems.

[0046] Returning to FIG. 1, the planetary train 16, which can be asimple planetary gear set or a compound planetary gear set, such asSimpson gear set or Ravigneaux gear set, has at least three concentricrotation members 38, 40, and 42 referred to as the planet carrier, ringgear, and sun gear, respectively. A set of planetary gears 44 mesheswith the second rotation member (ring gear 40) and the third rotationmember (sun gear) 42 in a conventional manner for a planetary train.

[0047] The first rotation member (planet carrier) 38 is operativelyconnected to the engine output shaft 17 for receiving power from thedriving engine 12. The connection between the first member 38 and theengine output shaft 17 may be direct, or may be through a conventionalgear train, one or more conventional clutches, or one or moreconventional torque converters (not shown).

[0048] The second concentric rotation member (ring gear) 40 of theplanetary train 16 is operatively connected to the mechanicaldifferential gearbox 20 by an input shaft 46 for delivering power to themechanical differential gearbox 20. The connection between the secondconcentric rotation member 40 and the input shaft 46 may be directlythrough the mechanical clutch 22, or may be through one or moreconventional clutches in conjunction with gear trains, belts, or chains(not shown).

[0049] The third concentric rotation member (sun gear) 42 of theplanetary train 16 is operatively coupled to the primary electric motor18, either directly or through one or more conventional gear trains (notshown).

[0050] The primary electric motor 18 and each of the secondary electricmotors 30, 32 are coupled to a control unit 48. The control unit 48regulates the flow of electrical power received from or delivered to theprimary electric motor 18 and each of the secondary electric motors 30and 32, and controls the operational speed of each motor 18, 30, and 32.An electrical energy storage unit 50 is further coupled to the controlunit 48, and is configured to store or dispense electrical power asdirected by the control unit 48.

[0051] During operation of a vehicle 100 equipped with the wheel drivesystem 10 of the present invention, multiple operational modes areprovided, including neutral, parking, engine start, forward, and reverseoperational modes.

[0052] In the neutral mode of operation, the mechanical clutch 22 isdisengaged and the secondary motor 30 and 32 are switched off.

[0053] In the parking mode of operation, the mechanical clutch 22 andthe brake system 24 are engaged. The primary motor 18 is either in a“free wheeling” state, switched off, or in a charging state. Secondarymotor 30 and 32 are switched off.

[0054] In the engine start mode of operation, mechanical clutch 22 isdisengaged, and the brake system 24 is engaged. The control unit 48directs electric power from the energy storage unit 50 to the primarymotor 18, which is utilized to start the driving engine 12 by rotatingthe output shaft 17 through the planetary gear train 16.

[0055] In the forward operational mode, the vehicle may be eitherstationary (i.e. idling) or in motion. When the vehicle is stationary,the primary motor 18 is in a freewheeling state, providing substantiallyzero torque as required to balance the driving engine 12. Each of thesecondary motors 30 and 32 are stationary, but are ready to provideinitial launch torque to the associated driven wheels 14.

[0056] Turning to the graphical representations of speed, torque, andpower shown in FIGS. 2, 3, and 4, when the control unit 48 is instructedby an operator to move the vehicle forward, the control unit 48 signalsthe primary motor 18 to increases the output torque required to balancethe increasing torque of the driving engine 12. Consequently, theprimary motor 18 functions as an electrical generator, providingelectric power to each of the secondary motors 30 and 32 through theinterconnected control unit 48. Each of the secondary motors 30 and 32operators to convert the received electric power to mechanical power,driving each associated driven wheel 14 together with mechanical powerprovided by driving engine 12 through planetary train 16 to each of theshafts 26 and 28. During this period, the driving engine 12 increasesoutput torque till a maximum rated power level is reached, at whichpoint the operational speed of the primary motor 18 begins to decrease,as is seen in FIG. 2.

[0057] The operational speed of the primary motor 18 continues todecrease as the forward speed of the vehicle 100 increases. Theoperation speed of the primary motor 18 eventually reaches zero, atwhich point the primary motor 18 ceases to generate electrical power, asseen in FIG. 4. As shown in FIGS. 2 and 4, this is considered a nodepoint at which the drive system 10 of the present invention achievesmaximum power transmission efficiency. Continued operation beyond thenode point results in the primary motor 18 reversing rotationaldirection and increasing operational speed. The reversed rotationaldirection of the primary motor 18 results in the primary motor 18receiving electric power generated by each of the secondary motors 30and 32.

[0058] The plots shown in FIGS. 2, 3, and 4 assume the vehicle 100 istraveling straight in the forward direction and that each of the drivenwheels 14 experience equal traction forces with the road surfaces.Further, it is assumed that the driving engine 12 is under full load andreaches a desired maximum operational speed.

[0059] When road surface conditions change, the maximum availabletraction experienced by each driven wheels 14 may vary. It has beenobserved that matching the individual drive torque applied to eachdriven wheel 14 with the road surface condition, as represented by thetraction experienced by each driven wheel 14, can effectively preventthe driven wheels 14 from slipping. Individual drive torque is matchedto the road surface conditions by redirecting power from a slippingdriven wheel 14 to a non-slipping driven wheel 14, thus enhancingoverall drive power.

[0060] Under uniform road surface conditions, it is often desirable tovary the driving torque supplied to each of the driven wheels 14 toenhance the dynamic performance of the vehicle 100, for example, whenturning FIGS. 5 and 6 plot torque and power curves under forwardoperational conditions with the driving engine 12 under one-half loadand operating at maximum operational speed for a vehicle 100, assumingthe driven wheel 14 on the one side receives twice as much of thedriving torque as the driven wheel 14 on the opposite side of thevehicle 100.

[0061] In the reverse mode of operation, the drive system 10 of thepresent invention operates as a series hybrid-drive to avoid internalpower circulation. In a series hybrid-drive, all power supplied to thedriven wheels 14 is provided by the secondary motors. The mechanicalclutch 22 is disengaged, and the braking system 24 engaged. The primarymotor 18 operation is controlled by the control system 48 to operate ina generator state, converting power received from the driving engine 12through the planetary train 16 into electrical power. The generatedelectrical power is routed through the control system 48 to thesecondary motors 30, 32 where it is converted to mechanical power,driving the driven wheels 14 of the vehicle 100 in the reversedirection.

[0062] Turning to FIG. 7, a first alternate embodiment of the drivesystem 10 of the present invention is shown. A direction selectingdevice 102 is coupled to an input shaft 45 of the mechanicaldifferential gearbox 20, facilitating a parallel hybrid-driveconfiguration of the drive system 10 during both forward and reversemodes of operation. In a parallel hybrid-drive configuration, eachdriven wheel 14 receives power from both the mechanical power pathwayand from the electrical power pathway. The direction selecting device102 consists of a pinion gear 104, a pair of bevel gears 106A and 106Bmeshed with the pinion gear 104, and a pair of clutches 108A and 108B.

[0063] To operate in the forward mode, one of the clutches 108A, 108B isengaged, and the other is disengaged. To operate in the reverse mode,the clutches 108A and 108B reverse their respective engagement status.Changes in between the vehicle forward and reverse operational modespreferably occur under zero speed and zero torque conditions such thatthe transition occurs smoothly. Using the direction selection device102, operational characteristics of the drive system 10 in the reversemode of operation are a mirror image of the operational characteristicsof the drive system 10 in the forward mode of operation.

[0064]FIG. 8 through FIG. 13 each illustrate alternate embodiments ofthe drive system 10 of the present invention. In FIG. 8, each of thesecondary motors 30, 32 are coupled to a set of driven wheels 14 whichare not mechanically coupled to the mechanical differential gearbox 20,thereby providing a four-wheel drive system.

[0065] In FIG. 9, two additional secondary motors 200A and 200B areemployed in the drive system 10. These secondary motors 200A and 200Bare coupled to a set of driven wheels 14 through associated wheel outputshafts 201A and 201B, which are not mechanically coupled to themechanical differential gearbox 20, thereby providing a four-wheel drivesystem. Each of the secondary motors 30, 32, 200A and 200B areinterconnected to each other and to the energy storage system 50 thoughthe control unit 48.

[0066] Further variations on the alternate embodiment of the drivesystem 10 shown in FIG. 9 are illustrated in FIGS. 10, 11, and 12 wheretwo or more of the driven wheels 14 (i.e. front wheels, rear wheels, orall four driven wheels 14) are coupled to the drive system 10 throughone or more associated conventional speed reduction devices 202. Theconventional speed reduction device 202 can be as simple as a piniongear 204 meshing with a bull gear 206, or a planetary gear train thatcontains a sun gear 204A or 204B, a ring gear 205A or 205B, and a set ofplanets along with a carrier 206A or 206B. The pinion gear 204 or sungear 204A, 204B is connected to the associated secondary motor 30, 32,200A or 200B, and bull gear 206, ring gear 206A, or planet carrier 206Bis connected to the associated wheel output shafts 34, 36, 201A or 201B.

[0067] An additional alternate embodiment of the drive system 10 shownin FIG. 13 illustrates the intermediate output shafts 26 and 28 from themechanical differential gearbox 20 coupled directly to the associatedwheel output shafts 34 and 36. The secondary motors 30 and 32 arecoupled to the respective wheel output shafts 34 and 36 throughconventional speed reduction units 210. Each speed reduction unit 210consists of a planetary gear set 212 each having at least a sun gear214, a ring gear 216, a planetary gear 218 and a planet carrier 220. Atleast one of the planetary members in the planetary gear set 212 isgrounded, and at least one of the remaining members in the planetarygear set 212 is coupled to the an associated secondary motor 30 or 32.

[0068] Graphical representations of engine and motor torque, speed, andpower as a function of vehicle wheel speed for the variations of thedrive system 10 depicted in FIGS. 11-13 are shown in FIG. 14 throughFIG. 16.

[0069] In view of the above, it will be seen that the several objects ofthe invention are achieved and other advantageous results are obtained.As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

1. A vehicle drive system for a vehicle having a driving engine with anoutput shaft and at least one set of driven wheels, comprising: aplanetary gear train coupled to the driving engine output shaft; aprimary electric motor coupled to said planetary gear train; amechanical differential gearbox coupled between said planetary geartrain and each of said driven wheels in said at least one set of drivenwheels; and at least a pair of secondary electric motors, each of saidsecondary electric motors coupled to one of said driven wheels in saidat least one set of driven wheels; wherein said planetary gear train andsaid mechanical differential gearbox define a mechanical power path fromsaid driving engine to said driven wheels; and wherein said planetarygear train, said primary electric motor, and each of said secondaryelectric motors define an electrical power path from said driving engineto said driven wheels.
 2. The vehicle drive system of claim 1 furtherincluding: a mechanical clutch interposed between said mechanicaldifferential gearbox and said planetary gear train; and a braking systemoperatively coupled to said planetary gear train; and wherein saidmechanical clutch and braking system cooperate to regulate a flow ofmechanical power to said mechanical differential gearbox.
 3. The vehicledrive system of claim 1 further including an electronic controllerinterposed between said primary electric motor and each of saidsecondary electric motors; and wherein said electronic controllerregulates a flow of energy to and from said primary electric motor andsaid secondary electric motor.
 4. The vehicle drive system of claim 3further including an energy storage system coupled to said electroniccontroller; wherein said electronic controller regulates a flow ofenergy to and from said energy storage system.
 5. The vehicle drivesystem of claim 1 further including a direction selection deviceoperatively coupled between said planetary gear train and saidmechanical differential gearbox.
 6. The vehicle drive system of claim 1wherein said vehicle includes a first set of driven wheels and a secondset of driven wheels, said mechanical differential gearbox coupledbetween said planetary gear train and each of said driven wheels in saidfirst set of driven wheels; and each of said secondary electric motorscoupled to one of said driven wheels in said second set of drivenwheels.
 7. The vehicle drive system of claim 6 further including asecond pair of secondary electric motors, each of said second pair ofsecondary electric motors coupled to one of said driven wheels in saidfirst set of driven wheels.
 8. The vehicle drive system of claim 1further including a plurality of speed reduction units, each of saidplurality of speed reduction units coupled between said mechanicaldifferential gearbox and one of said driven wheels in said at least oneset of driven wheels.
 9. The vehicle drive system of claim 8 whereineach of said plurality of speed reduction units is further coupled toone of said pair of secondary electric motors, said speed reduction unitincluding: a planetary gear set having a sun gear, a ring gear, aplanetary gear, and a planet carrier; and wherein at least one componentof said planetary gear set is grounded.
 10. A method for deliveringpower from a driving engine to the driven wheels of a vehicle through avehicle drive system having a planetary gear train coupled to a drivingengine output shaft, a primary electric motor coupled to said planetarygear train, a mechanical differential gearbox coupled between saidplanetary gear train and each of said driven wheels in said at least oneset of driven wheels, and at least a pair of secondary electric motors,each of said secondary electric motors coupled to one of said drivenwheels in said at least one set of driven wheels, comprising:selectively utilizing a portion of mechanical power received from saiddriving engine to drive said set of driven wheels; selectivelyconverting a portion of mechanical power received from said drivingengine into electrical power; routing said electrical power between saidprimary motor and at least one of said secondary electric motors;utilizing said electric power to provide mechanical power at said drivenwheel associated with said at least one secondary electric motor in afirst operational state, and utilizing said electrical power to drivesaid primary motor in a second operational state.
 11. The method ofclaim 10 wherein the step of converting a portion of mechanical powerreceived from said driving engine into electrical power includesbalancing an output torque of said primary electric motor to an inputtorque received from said driving engine responsive to said drivingengine operating below a predetermined power level.
 12. The method ofclaim 11 wherein the step of converting a portion of mechanical powerreceived from said driving engine into electrical power includesincreasing an output torque of said primary electric motor responsive tosaid driving engine operating at predetermined power level and saiddriven wheels rotating below a predetermined speed.
 13. The method ofclaim 11 wherein the step of converting a portion of mechanical powerreceived from said driving engine into electrical power includesoperating said primary electric motor in a reverse direction at anincreasing rotational speed and extracting electrical power from atleast one of said secondary electric motors responsive to said drivingengine operating at predetermined power level and said driven wheelsrotating above a predetermined speed.
 14. The method of claim 10 furtherincluding the steps of: detecting slippage between a road surface and adriven wheel in said at least one set of driven wheels; redirectingdriving power from said slipping driven wheel to an remaining drivenwheel in said one set of driven wheels.